The oil and gas industries utilise a technology known as hydraulic fracturing or “fracking”. This normally involves the pressurisation with water of a system of boreholes in oil and/or gas bearing rocks in order to fracture the rocks to release the oil and/or gas.
In order to achieve this pressurisation, valves may be used to separate different sections of a borehole system. These valves are referred to as downhole valves, the word downhole being used in the context of the disclosure to refer to an article that is used in a well or borehole.
One way of forming such valves involves the use of spheres of material known as fracking balls to seal off parts of a borehole. Fracking balls may be made from aluminium, magnesium, polymers or composites.
A problem with the use of fracking balls relates to how they are removed once the fracking operation has been completed in order to allow fluid to flow through the well or borehole. One way of doing this is to drill through the fracking ball. However, this type of drilling process can hamper production, as well as being expensive, difficult and therefore undesirable.
One proposed solution to this problem has been to form the fracking ball from a material that will dissolve or corrode under the conditions in the well or borehole. An issue that needs to be considered in relation to such corrodible articles is ensuring that they corrode at a rate which allows them to remain useable for the time period during which they are required to perform their function, but that allows them to corrode or dissolve afterwards.
Degradable polymers have been used in order to provide a corrodible article for use in such methods. However, these polymers do not generally have particularly high mechanical strength.
An alternative corrodible article is described in U.S. Pat. No. 8,425,651 in the name of Xu et al. This document describes a powder metal composite comprising a nanomatrix, preferably made of Al or Ni or a combination thereof, in which are dispersed a plurality of first particles, a plurality of second particles and a solid-state bond layer. The first particles comprise Mg, Al, Zn or Mn, or a combination thereof, and the second particles comprise carbon nanoparticles. The composite may be produced by forming a powder mixture of the required components and then applying temperature and pressure to the powder to sinter and deform (but not melt) the composite in order to form a powder composite. A problem with such powder metallurgical methods is that they are complicated and expensive.
A further corrodible article is described in US patent application publication no 2012/0318513 in the name of Mazyar et al. In this document, the corrodible article is described as having a corrodible core and a metallic layer covering the core. The core material is described as being a magnesium alloy. However, it appears that the combination of magnesium and one or more other materials in a form which is not an alloy is also intended to be covered by the use of the term “alloy” in Mazyar et al. For example, this document refers to alloys of magnesium with tungsten, whereas it is actually not technically feasible to form a magnesium-tungsten alloy. Similarly, Mazyar et al also mentions powders of magnesium coated with a metal oxide as being useful for forming the core, which again would not be magnesium “alloys”. Thus, Mazyar et al appears to utilise the term “magnesium alloy” to mean any way in which magnesium and another metal are combined. The metallic layer is described as including aluminium or nickel.
Although casting, forging and machining are described in Mazyar et al, these are only mentioned in very general terms (e.g., method steps and heating temperatures are not stated) and the structure of the resulting materials is not described. In addition, the preferred method of forming the corrodible article is by compressing the powder into the desired shape, for example by cold compression using an isostatic press. As noted above, such powder metallurgical methods are complicated and expensive. In addition, the resulting powder composites can have poor mechanical properties.
Thus, there is a need in the oil and gas industries to provide a corrodible article which provides the desired corrosion characteristics, whilst also having improved mechanical properties, and at a lower cost than can currently be achieved. It is also advantageous for the corrodible article to have a relatively low density (for example, compared to metals in general). This disclosure seeks to ameliorate these problems and to achieve these effects.
Many features, advantages and a fuller understanding of the disclosure will be had from the accompanying drawings and the Statement of the Disclosure that follows. It should be understood that the following Statement of the Disclosure describes the subject matter of the disclosure and presents specific embodiments that should not be construed as necessary limitations of the broad invention as defined in the claims.